I just switched to KDE for testing and I found something quite funny:
The KDE Kickoff Menu showes up after about 1 - 2 seconds when it is klicked on.
(Klick 1 - 2 Seconds (System Freezes) Kickoff appears.)
The System freezes completly.
Not only the menu, everything.
I switched in the effect to monitor Framerate, this is when I discovered the freezing.